Welcome to the site. The peice with hole in it is part of the back of the unit, it's only the outside trim that is removeable
They have one on this site, have a look. I would just the center out and add a chunk of wood to mount the speaker on.http://www.familyhandyman.com/walls/...iding/view-all

Aha! Thanks Neal and Wow. I was beginning to think it was "fixed" and puzzling how to get something substantial in its place. I like the idea of keeping the cover -- for extra weather protex for the wood behind it... thanks again.

All righty, yes, pulling the bezel gets me access -- but you folks knew that already. But that exposed another question: the vinyl courses are so tight to the block that I can't get at the fasteners for the block.
I'd prefer to not cut the face of cover, but rather, remove it, mount wood behind it, replace the cover, mount the speakers.

I presume I need to release some of the vinyl courses enough so I can get at the block fasteners. I know vinyl siding has some funky tools -- which do you think I'd need for this?

The right way is the way your trying to do it, remove the whole thing and add 3/4 plywood or OSB in the hollow area. That same size block can be bought that's solid faced so you could drill a smaller hole for the wiring.
Often times I can not find my Zipper tool so I just use my flat bar. Going to need one anyway to remove the roofing nails.